1. Field of the Invention
This invention relates to electromechanical or electroacoustical transducers, and more specifically to such transducers which convert between electrical and mechanical (including acoustical) energy by means of the piezoelectric effect available in thin high polymer films, such as polyvinylidene fluoride, which have been uniaxially oriented and electrically polarized, and have surface electrodes thereon for connection to electrical circuits.
2. Description of the Prior Art
Since its initial discovery through the work of Kawai, reported in 8 Japan. J. Appl. Phys. 975-976 (1969), the development of the piezoelectric effect in thin high polymer films by means of uniaxial orientation and subsequent electrical polarization has resulted in electromechanical coupling coefficients exceeding 15%. As with the work of Kawai this work has concentrated on polyvinylidene fluoride (abbreviated PVF.sub.2), but improved materials can be expected in the future, as well as further improvement in the coupling coefficient of PVF.sub.2.
The application of such films to practical transducers has been hindered by the unusual mechanical characteristics of the films compared with conventional piezoelectric materials and the forms in which they have been available. That is, the thinness and the low elastic modulus of the films present new problems in transducer structure, while at the same time these same characteristics, combined with the low mass/area ratios of the films, offer the potential for greatly improved transducer performance in several areas of application.
In applying piezoelectric films to use in electromechanical or electroacoustic transducers, the recent art has arranged the film in a primitive shell configuration, such as a cylinder or a portion of a cylinder, to transform between (1) strain in the film along the uniaxial direction (which corresponds to the largest piezoelectric effect) and tangent to the film surface, and (2) the motion normal to the film surface necessary if direct electroacoustic transduction and the accompanying low mass/area ratio are to be achieved.
In a cylindrical shell, for example, an acoustic pressure difference between the surfaces of the piezoelectric film is supported by the arch of the cylindrical form and is transformed in part into stress and strain tangential to the film and in the uniaxial direction. Because of the electromechanical coupling coefficient k of the film, a signal voltage is generated between the electrodes on the film. Conversely, if an electrical signal is applied to the electrodes, strain is generated by the piezoelectric effect in the uniaxial direction, and the cylindrical form of the film changes by deflections normal to its surface, resulting in the output of acoustical energy.
However, the film is very thin, typically 8 to 30 microns, and its elastic modulus is low, typical of organic polymers. Thus elastic instability can set in at very low pressure differences, resulting in unacceptable harmonic distortion, failure of the frequency response to be approximately independent of signal level, and lack of reproducibility of performance characteristics in general. For example, an airborne shock wave or other acoustic overload can irreversibly damage or change the form of the film even to the point of substantially reversing its curvature. For these reasons, the level of elastic stability attainable with this configuration is insufficient for practical use.
One technique that has been used to supply elastic stability is that of mechanically biasing the transducer, i.e., by providing a static pressure on one side of the film so as to produce tension in the film in the uniaxial direction. Typically this pressure is supplied mechanically by a pad of flexible foam, held under compression by a perforated backing plate to cause it to exert pressure on the underside of the curved film, which consequently is placed under static tension. The acoustically active vibration of the film adds a dynamic component to the tension, but elastic stability is more than assured if the total tension does not reverse sign to become compressive.
The outstanding disadvantage of using compressed foam to develop a mechanical bias to procure elastic stability is the deleterious effect of the incremental stiffness of the foam on the effective electroacoustic coupling coefficient of the transducer. In most practical transducers there will be an air volume coupled to one side of the film and the acoustical compliance of this space is one of the most basic parameters that restricts the performance of the transducer. The incremental stiffness of the foam markedly decreases the effective acoustical compliance of this space, without any corresponding reduction in the space occupied by the transducer. Furthermore, the foam adds effective mass to that of the film, and most of the film's low mass/area ratio advantage is lost.
Another prior art solution providing mechanical bias by means of the tractive force of an electrical field is disclosed in U.S. Pat. No. 3,894,198, but this device requires a combination of a piezoelectric thin film transducer with an electrostatic transducer that is either externally or electret polarized.
Despite these efforts of the prior art, no transducer structures using high polymer piezoelectric films have approached in performance characteristics the full potential offered by the intrinsic properties of such films.